Investigation of a novel stepped-current vis-a-vis conventional resistance spot welding of dual-phase 780 (DP 780) steel

One of the challenges in resistance spot welding (RSW) of advanced high-strength steels (AHSS) is the narrow processing window or welding current range in which adequate nugget diameter for the desired pull-out failure can be achieved. In this work, a novel stepped-current RSW (SC-RSW) is systematically compared with the conventional RSW (C-RSW) for high-strength dual-phase steel (DP 780) concerning the evolution of nugget diameter, microstructure, hardness profile and mechanical performance under different modes of loading, viz., tensile-shear (TS), Coach-peel (CP) and cross-tension (CT). It is revealed that the welding current range increased from 2 kA in C-RSW to 3 kA in the novel SC-RSW process. Further, for any given average welding current, the nugget diameter was significantly higher for the SC-RSW process than for C-RSW. The evolution of nugget diameter in both welding processes is clearly understood based on finite element analysis (FEA) and in-situ monitoring of dynamic resistance. In a wide range of welding currents, especially at lower average current values, SC-RSW enhanced load-bearing capacity and energy absorption under different loading conditions. At adequate welding parameters, the desired pull-out failure occurred where the crack initiated at the softened subcritical heat affected zone due to localized softening of martensite and propagated in the thickness direction of the sheet. The maximum load-bearing capacity was achieved in the TS loading mode for a given welding parameter. In contrast, the highest energy absorption could be achieved in the cross-tension loading condition. This study can pave the way for the practical design of the weld cycle for different AHSS suitable for industrial adaptation.